1. Field of the Invention
The invention generally relates to positioning systems and, more specifically, to methods and systems of classifying WLAN access points in a WLAN positioning system. The invention further relates to calculating the quality of estimation of characteristics of the WLAN access points and scaling a reference database accordingly.
2. Discussion of Related Art
Position determination is the main component of navigation systems and any Location Based Services (LBS). Proliferation of WLAN access points in recent years created a blanket of WLAN radio waves everywhere. Therefore, almost in any place, there is a great possibility of detecting WLAN radio waves, especially in urban areas. The exponential growth of WLAN, and the fact that they can be found almost everywhere, initiated an idea of leveraging them for a metropolitan positioning system for indoor and outdoor areas. In a metropolitan WLAN positioning system, location of WLAN access points are used as reference points, and the Received Signal Strength (RSS) of a WLAN access point is used as an indicator of a distance of an end user from the WLAN access points that the user detects at any time. By knowing the distance of the end user from WLAN access points, location of the end user can be determined. Translating receiver Receive Signal Strength to distance relies on assuming a specific radio channel model. Ideally, if the radio channel model was exactly known, the exact distance of the end user to WLAN access points could be found.
Outdoor and indoor WLAN based positioning systems have been explored by couple of research labs, but none of them included speed and bearing estimation in their system. The most important research efforts in this area have been conducted by PlaceLab (www.placelab.com, a project sponsored by Microsoft and Intel), University of California San Diego ActiveCampus project (ActiveCampus—Sustaining Educational Communities through Mobile Technology, technical report #CS2002-0714), and the MIT campus wide location system, and it was evaluated through several small projects at Dartmouth college (e.g., M. Kim, J. J. Fielding, and D. Kotz, “Risks of using AP locations discovered through war driving”).
There have been a number of commercial offerings of Wi-Fi location systems targeted at indoor positioning. (See, e.g., Kavitha Muthukrishnan, Maria Lijding, Paul Havinga, Towards Smart Surroundings: Enabling Techniques and Technologies for Localization, Proceedings of the International Workshop on Location and Context-Awareness (LoCA 2005) at Pervasive 2005, May 2005, and Hazas, M., Scott, J., Krumm, J.: Location-Aware Computing Comes of Age. IEEE Computer, 37(2):95-97, February 2004 005, Pa005, Pages 350-362.) These systems are designed to address asset and people tracking within a controlled environment like a corporate campus, a hospital facility or a shipping yard. The classic example is having a system that can monitor the exact location of the crash cart within the hospital so that when there is a cardiac arrest the hospital staff doesn't waste time locating the device. The accuracy requirements for these use cases are very demanding typically calling for 1-3 meter accuracy.
These systems use a variety of techniques to fine tune their accuracy including conducting detailed site surveys of every square foot of the campus to measure radio signal propagation. They also require a constant network connection so that the access point and the client radio can exchange synchronization information similar to how A-GPS works. While these systems are becoming more reliable for indoor use cases, they are ineffective in any wide-area deployment. It is impossible to conduct the kind of detailed site survey required across an entire city and there is no way to rely on a constant communication channel with 802.11 access points across an entire metropolitan area to the extent required by these systems. Most importantly outdoor radio propagation is fundamentally different than indoor radio propagation rendering these indoor positioning algorithms almost useless in a wide-area scenario. The required accuracy of indoor WLAN based positioning systems, makes it hard to use radio channel modeling and it is considered as a research topic in that domain. In addition, none of the WLAN based positioning systems to date have distinguished between access points, and current methods treat all WLAN access points the same.
FIG. 1 depicts a Wi-Fi positioning system (WPS). The positioning system includes positioning software [103] that resides on a computing device [101]. Throughout a particular coverage area there are fixed wireless access points [102] that broadcast information using control/common channel broadcast signals. The client device monitors the broadcast signal or requests its transmission via a probe request. Each access point contains a unique hardware identifier known as a MAC address. The client positioning software receives signal beacons from the 802.11 access points in range and calculates the geographic location of the computing device using characteristics from the signal beacons. Those characteristics include the unique identifier of the 802.11 access point, known as the MAC address, and the strengths of the signal reaching the client device. The client software compares the observed 802.11 access points with those in its reference database [104] of access points, which may or may not reside on the device as well. The reference database contains the calculated geographic locations and power profile of all the access points the gathering system has collected. The power profile may be generated from a collection of readings that represent the power of the signal from various locations. Using these known locations, the client software calculates the relative position of the user device [101] and determines its geographic coordinates in the form of latitude and longitude readings. Those readings are then fed to location-based applications such as friend finders, local search web sites, fleet management systems and E911 services.